Left my iPhone 4 for the S3. Had an iPhone for 3-4 years. I was just getting tired of the small screen. And I HATED iTunes. My iPhone never synced with it because every time I sync, it would delete a bunch of my apps or just reset it.
It took a little getting used to my S3. I also dislike the email app in S3 because I have to "load more detail" or something like that every time. It is trouble some for me. With the iPhone, the email just showed up correctly. I have set my email to load everything (images and no file size limit to no avail). But, I am getting used to the email client.
I do love that I have so many options with Android though. Since I got the S3, I have spent more money at the Play Store than when I had my iPhone. With the iPhone App Stores, the apps just didn't seem like it was worth it to buy. And I only used the free ones. With the S3 or Play Store, all the free ones I used to use are here and there are just more apps that I wanted to buy.
I might go back to the iPhone if they improve their screen and flexibility, but most of all, I NEED that microSD card slot. That's one of the best features. I also love the fact that the S3 has USB OTG. I can copy files to a flash drive and I can just plug that flash drive to the S3 and use it. That's awesome.
iPhone looks slicker but that's mostly it. I am enjoying the S3. But if a new Nexus comes out this year, I might have to get that instead.